In recent years fiber optic gyroscopes (FOGS) have become widely used in several technologies to sense the rotation and angular orientation of various objects, such as aerospace vehicles. A FOG typically includes an optical fiber, often several kilometers in length, wound in a coil about an axis of rotation (i.e., the rotation to be sensed). Light is injected in opposite directions through the coil and directed onto a photo detector. If the coil is rotated about the axis, the effective optical path length for the light traveling in one direction in the coil is increased, while the path length is decreased for the light traveling in the opposite direction (See FIG. 1).
The difference in path length introduces a phase shift, known as the Sagnac Effect, between the light waves traveling in opposite directions. As a result, an interference pattern is detected by the photo detector, which indicates that the FOG is experiencing rotation. The output signal from the photo detector typically follows a cosine function. That is, the output signal depends on the cosine of the phase difference between the two waves. Therefore, because the cosine function is even, the rate of change near zero is very small, resulting in poor sensitivity for low rotation rates. To improve sensitivity, the waves are often modulated by a particular voltage to generate a phase difference deviating from zero. The actual phase generated by the modulation voltage is referenced to a voltage Vπ that changes the light phase by 180° (or π radians). The phase modulation reference voltage Vπ depends on the wavelength and environmental factors, such as temperature, humidity, and pressure.
The amplitude of the Sagnac phase shift caused by a rotation rate determines the FOG “scale factor,” which numerically relates the detected phase difference to the actual rotation being experienced by the FOG. As the fiber sensing coil length and diameter are increased, the scale factor also increases, and as a result, the FOG is more sensitive to rotation. The FOG scale factor also depends on the wavelength of the light propagating in the fiber sensing coil.
In the Interferometric Fiber Optic Gyroscope (IFOG), vibration can cause intensity fluctuation through components in the optical signal path. Vibration also causes phase fluctuation through physical rotation of the IFOG sensing coil. Rectified bias error is generated when the intensity and phase modulations are synchronous. Both Navy Submarine Launched Ballistic Missile (SLBM) and Air Force Intercontinental Ballistic Missile (ICBM) guidance programs require IFOG to exhibit minimal rectified bias error under severe vibration environmental situations, such as during booster phase and stages separation (see FIGS. 2 and 3).
Therefore, there exists a need for a technique to suppress vibration-induced intensity modulation and thus reduce the vibration-induced bias error.